Field of the Invention
The present invention relates to a silicon dioxide film formed by the plasma enhanced deposition of silicon dioxide through a reaction of TEOS oxide (tetraethoxysilane, tetraethylorthosilicate) in an oxygen plasma and a method for preparing the silicon dioxide film. The silicon dioxide film can be used in thin film transistor (TFT) devices. The method includes controlling the deposition rate of TEOS oxide on a substrate by pulsing the radio frequency (RF) power supply which results in a thin dielectric film with superior film qualities.
Plasma enhanced chemical vapor deposition (PECVD) is a key technology for the production of display devices. For Active Matrix Liquid Crystal Displays (AMLCDs), PECVD is used to deposit the active layers of amorphous Si (a-Si), n+Si and SiN, as well as interlayer dielectric layers and passivation layers. Performance of the thin film transistors used in the arrays of AMLCDs is critically dependent on the conditions and characteristics of the active layers of a-Si, n+Si and SiN dielectric. More recently, polysilicon AMLCDs have been developed, because this display technology simplifies the manufacturing of the displays by enabling the driver circuits to be integrated on the display panels. Again, PECVD technology is used to deposit the amorphous silicon precursor to polysilicon that is prepared by excimer laser annealing. In addition, PECVD technology is also utilized for the development and manufacture of Field Emitter Array (FED) devices.
The dominant active matrix technology is thin film transistors (TFTs) comprised of either amorphous silicon or polycrystalline silicon.
Thin films of amorphous silicon have been fabricated heretofore by a PECVD process comprising:
introducing a silane (SiH4) gas or a mixture of silane gas and hydrogen (H2) gas as a starting material into a film deposition (vacuum) chamber;
applying a high frequency power across a pair of facing electrodes to produce a plasma by electric discharge; and
exciting and decomposing the starting material gas to form a thin film of amorphous silicon on the surface of a substrate supported by one of the electrodes.
This film deposition process has been applied to the production of TFTs for use in LCDs or in flat panel displays. In general, the TFT elements are formed on a glass plate. A key point in the fabrication of LCDs of high quality is to deposit a film having a uniform film thickness. Thin films of amorphous silicon are no exception, and uniformity in the film thickness is recognized as an important factor to be fulfilled in their deposition.
During the manufacture of thin films of amorphous silicon, it is known that the discharge between the facing electrodes in PECVD can be effected in either of two ways: one is a continuous discharge method; the other is an intermittent discharge method in which a square wave amplitude-modulated discharge is used.
The continuous discharge method is characterized in that it enables the deposition of high-quality films of amorphous silicon while maintaining the substrate at a relatively low temperature of, e.g., about 250xc2x0 C. When depositing a thin film of amorphous silicon on a glass plate having a large area (e.g., 20xc3x9720 cm2), however, this method is problematic because it is difficult to obtain thin films of uniform thickness. In addition, semiconductor films formed by methods that achieve a higher-rate film formation by supplying a larger high-frequency power and/or supplying a larger amount of material gas may contain a large amount of polysilane powder if the pressure is not controlled properly, thereby resulting in a product having a low yield.
The intermittent discharge method comprises applying a square wave amplitude-modulated radio frequency (rf) between facing electrodes. The other basic film deposition conditions (e.g., pressure, substrate temperature, and composition and flow rate of the starting material gas) may be the same as those used in the continuous discharge method. The square wave amplitude-modulated discharge method was initially proposed by Overzet et al., see L. J. Overzet et al., Appl. Phys. Lett., 48(11):695-97 (1986), which discloses a process that enhances deposition in low power rf discharges. This method has been studied in further detail by Watanabe et al. of Kyushu University. For example, Watanabe et al., AppI. Phys. Lett., 53(14): 1263-5 (1988), discloses the use of a modulated rf discharge of silane diluted with helium to improve the quality of xcex1-Si:H films.
Intermittent discharge methods are known to reduce polysilane powder generation within the reaction apparatus when depositing an a-Si: H film. See Watanabe et al., Appl. Phys. Lett., 57(16): 1616-18 (1990).
Denisse et al. of Utrecht State University have reported the application of an intermittent discharge method to a process for depositing a thin film of SiOxNy. See Denisse et al., J. Appl. Phys. 60(7):2536-42 (1986).
U.S. Pat. Nos. 5,437,895 and 5,618,758 disclose a process for forming a silicon-containing thin film on an insulating substrate using PECVD while intermittently generating a high frequency discharge.
U.S. Pat. No. 5,298,290 discloses that the parameters for a plasma polymerization method depend very strongly on the gaseous compounds used. It is known from European patent reference EP A 207 767, to pulsate rf plasma, during a surface treatment by plasma enhanced reactive processes. Without any specific selection, a large number of different materials are proposed for processing in this reference, e.g., Si3N4, TiO2, Al2O3, BN, SiO2, B4C, SiC, HC, TiC, TiN, BP. All of these coating materials are not produced by polymerization. The reference discloses that how the process behaves, depends on plasma modulation, and to a large extent, on the specific gaseous compound supplied for the coating operation.
Silicon dioxide films formed from the oxidation of TEOS oxide (tetraethoxysilane, tetraethylorthosilicate) are commonly used in the semiconductor industry as intermetal-dielectric films. There is an interest in using TEOS as a gate oxide for thin film transistor (TFT) devices. For example, U.S. Pat. No. 5,462,899, the entire contents of which are incorporated herein by reference, discloses the use of a continuous discharge method to form a silicon dioxide film on a substrate using TEOS as a principal reagent. However, a PECVD process based on a continuous discharge method is disadvantageous in that thinner films of TEOS oxide having uniform thickness cannot be achieved.
The present inventors have determined that one of the obstacles in using a continuous discharge method to form a silicon dioxide film using TEOS oxide is that the deposition rate could not be low enough to control the thickness of the gate oxide for applications in which the thickness of the silicon dioxide film is 500 xc3x85 or less without losing desirable qualities of important film properties.
Prior to the present invention, it had not been examined whether using intermittent discharge was suitable for use with TEOS oxide. The present inventors have discovered that PECVD processes using an intermittent discharge method enable the deposition of TEOS oxide to form a thin silicon dioxide film with uniform thickness over the surface of an insulating substrate. The thin films prepared by the method of the invention permit enhanced device and circuit performance, including increased switching speed, reduced power dissipation and smaller device and circuit areas.
The thin silicon dioxide films prepared by the method of the invention provide better MOS (metal-oxide-semiconductor) transistor performance.
Accordingly, an object of the present invention is to provide a process for forming a thin film of silicon dioxide of uniform thickness over the surface of an insulating substrate.
In accordance with the above object, the method of the invention involves decreasing the deposition rate of silicon dioxide, while at the same time achieving thinner films with high uniformity.
A method for producing a silicon dioxide film according to the present invention includes the steps of:
placing a TEOS precursor and oxygen in a plasma state;
decomposing the TEOS gas into active species;
reacting the TEOS with oxygen ions or radicals in the plasma;
depositing the active species on a substrate;
wherein energy for generating the TEOS oxide plasma is intermittently supplied at a supply time interval.
In one embodiment of the method for producing a silicon dioxide film of the invention, the silicon dioxide film is deposited at a rate at of less than 1100 angstroms/min, and the energy for plasma generation is intermittently supplied at a supply time interval of, for example, 1150 watts for 1 second and brought to at or about, for example, zero watts, for a time interval of, for example, 3 seconds. One skilled in the art would readily appreciate that deposition rate is hardware dependent. Thus, one skilled in the art would also readily appreciate that the required rate and the required pulse strength and on and off time as well as the required specific parameters to achieve a film having a desired film thickness are machine dependent. For example, by adjusting the parameters of the pulse (i.e., peak height or maximum power, peak floor or minimum power, peak duration (length of maximum power pulse) and duty cycle (number of maximum power pulses per unit time)), films of a desired thickness can be achieved. The method of the invention allows films thinner than 300 xc3x85, e.g., 200 xc3x85, or films thicker than 300 xc3x85 to be obtained.
In a preferred embodiment of the method for producing a silicon dioxide film of the invention, the power for plasma generation is intermittently brought down to at or about zero watts.
Thus, the invention described herein makes possible the advantages of providing a method for forming a high-quality silicon dioxide film suitable for use in thin film transistor devices. The film of the invention exhibits enhanced performance, e.g., faster switching speed, lower power dissipation and smaller device and circuit areas.
The present invention is illustrated below in greater detail with reference to non-limiting examples and accompanying drawings. It should be understood, however, that the present invention is not to be construed as being limited thereto.